Lehrer hat recht.
Schreibe mal erst alles ohne Wurzeln nur mit Exponenten
$$\frac{s}{t^2}*\frac{t^{\frac{10+u}{5}}}{s^{\frac{5-u}{5}}}*s^{\frac{5}{10}}*t^{\frac{8}{10}}*t^{\frac{1}{5}}: s^{\frac{5}{2}}$$
$$=\frac{s}{t^2}*\frac{t^{\frac{10+u}{5}}}{s^{\frac{5-u}{5}}}*t^2*s^{\frac{5}{10}}: s^{\frac{5}{2}}$$
$$=s*\frac{t^{\frac{10+u}{5}}}{s^{\frac{5-u}{5}}}*t^{-1}*s^{\frac{5}{10}}: s^{\frac{5}{2}}$$
$$=s*\frac{t^{\frac{10+u}{5}}}{s^{\frac{5-u}{5}}}*t^{-1}*s^{-2}$$
$$=\frac{t^{\frac{10+u}{5}}}{s^{\frac{5-u}{5}}}*t^{-1}*s^{-1}$$
$$=t^\frac{5+u}{5}*{s^{\frac{u-5}{5}}}*s^{-1}$$
$$=t^\frac{5+u}{5}*{s^{\frac{u-10}{5}}}$$